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Ganymede: Jupiter’s Oceanic Moon!

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From its discovery around Jupiter, to its composition, and what mysteries it may hold, and more! Join us as we explore Ganymede: Jupiter’s Oceanic Moon!
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8. The Finding And Naming Of Ganymede
At present, Jupiter has 79 moons, some of which have only been discovered recently. But in regards to Ganymede, it may have an origin that is far beyond the histories of the other moons.
Chinese astronomical records report that in 365 BC, Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Sh* Shen and Gan De together made fairly accurate observations of the five major planets.
On January 7th, 1610, Galileo Galilei observed what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and one body that turned out to be the combined light from Io and Europa; the next night he noticed that they had moved. On January 13th, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15th, Galileo came to the conclusion that the stars were actually bodies orbiting Jupiter. Thus, the discovery of the moons.
Galileo originally called Jupiter’s moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III, and IV. Galileo’s naming system would be used for a couple of centuries.
It wouldn’t be until the mid-1800’s that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
In mythology, Ganymede was a beautiful young boy who was carried to Olympus by Zeus (the Greek equivalent of the Roman god Jupiter) disguised as an eagle. Ganymede became the cupbearer of the Olympian gods. The Greek/Roman pantheons are the epicenter of many names of both planets and moons in our solar system.
7. Orbits and Rotations
Ganymede orbits Jupiter at a distance of 1,070,400 km, third among the Galilean satellites, and completes a revolution every seven days and three hours. Which indeed means that a “Day” on Ganymede is a week here on Earth, that would be something that would take some getting used to no doubt.
Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet, hence its day is seven days and three hours. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries.
Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times. Conjunctions (alignment on the same side of Jupiter) between Io and Europa occur when Io is at periapsis and Europa at apoapsis. Conjunctions between Europa and Ganymede occur when Europa is at periapsis. The longitudes of the Io–Europa and Europa–Ganymede conjunctions change with the same rate, making triple conjunctions impossible. Such a complicated resonance is called the Laplace resonance.
There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System; or that it developed after the formation of the Solar System. A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io’s orbit to expand (due to conservation of momentum) until it encountered the 2:1 resonance with Europa; after that the expansion continued, but some of the angular moment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede. Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.
6. Composition
Ganymede has three main layers. A sphere of metallic iron at the center (the core, which generates a magnetic field), a spherical shell of rock (mantle) surrounding the core, and a spherical shell of mostly ice surrounding the rock shell and the core.

Credits: Ron Miller
Credits: Nasa/ jpl/brown university/ voyager-iss/jpl-caltech/swri/msss
credits: Kevin Gill/ Brian Altmeyer/ Justin Cowart ( cc by 2.0)

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